1. Field of the Invention
The present invention relates to a method for preparing a cathode material for an alkali-sulfur cell, an alkali-sulfur cell or rather battery and an energy store.
2. Description of the Related Art
In order to produce batteries having a clearly greater energy density, in lithium-sulfur battery technology, at this time, (in brief: Li/S) is being researched. If the cathode of a lithium-sulfur cell consisted completely of elemental sulfur, in theory, an energy content of more than 1,000 Wh/kg could be achieved. However, sulfur is neither ionically nor electrically conductive, so that additives have to be added to the cathode which clearly lower the theoretical value. In addition, during the discharge of a lithium-sulfur cell, elemental sulfur is usually reduced to soluble polysulfides Sx2−. These are able to diffuse into regions such as the anode region, in which they are no longer able to participate in the electrochemical reaction of the subsequent charge/discharge cycles. In addition, polysulfides may be dissolved in the electrolyte which are not able to be reduced further. Therefore, in practice at this time, sulfur utilization, and, with that, the energy density of lithium-sulfur cells is clearly lower, and is estimated to be between 400 Wh/kg and 600 Wh/kg.
Various concepts exist for raising the sulfur utilization. Nazar et al. describe in Nature Materials, vol. 8, June 2009, 500-506, that carbon tubules favor retention of polysulfides in the cathode space and at the same time take care of sufficient conductivity.
Wang et al. describe in Advanced Materials, 14, 2002, No. 13-14, pp. 963-965 and Advanced Functional Materials, 13, 2003, No. 6, pp. 487-492, and Yu et al. describe in Journal of Electroanalytical Chemistry, 573, 2004, 121-128 and Journal of Power Sources, 146, 2005, 335-339 another technology in which polyacrylonitrile (abb.: PAN) is heated with an excess of elemental sulfur, the sulfur, on the one hand, being cyclized to form a polymer having a conjugated n-system while forming H2S polyacrylonitrile, and on the other hand, being bound in the cyclized matrix.